1. Field of the Invention
The present invention relates to a plastic can which is used as a container for food and drink.
2. Description of the Background:
Metal cans have conventionally been used as containers for food and drink. Recently plastic cans, plastic bottles, and plastic cups, have realized increasing use in the handling and storage of food and drink items because of their ease of production and disposal and because of their low cost. The plastic containers are now available in many varied forms because they can be easily prepared by injection molding, blow molding, and vacuum forming techniques. These containers are used for the handling and storage of a large variety of foods and drinks including edible oils, seasonings, and desserts.
In the past there has been significant activity in the development of food and drink handling containers, as is evident from the following list of patent applications: Japanese Patent Laid-open Nos. 11146/1983, 153629/1983, and 209561/1983, and Japanese Utility Laid-open Nos. 35315/1984 and 35333/1984. The containers described are plastic cans which usually have a can body made of laminated material. The cans meets different requirements depending on the particular application of the container. Plastic cans for foods and drinks are required to conform to food sanitation laws and must have such properties as water resistance, oil resistance, retortability, self-supporting, and gas barrier properties.
The can body of conventional plastic cans is illustrated by the partly enlarged sectional view of FIG. 4. (See, for example, Japanese Patent Laid-open No. 214627/1984.) In this particular example, cylinder (1') which forms the can body, is made up of a content protecting layer which is the laminated film (2) and a supporting layer. The content protecting layer is composed of the following five layers superposed one over the other as shown in FIG. 4:
A: Undrawn polypropylene layer, (70 .mu.m thick (innermost layer))
E: Adhesive layer (carboxylic acid-grafted polypropylene, 7 .mu.m thick)
F: Aluminum foil (9 .mu.m thick)
G: Undrawn polypropylene layer (30 .mu.m thick)
One edge (4) of the content protecting layer is folded back on itself and the facing surfaces are bonded together. The edge (5) of the adjacent layer overlaps and is bonded to the folded portion. The supporting layer of the can is made of the following four layers which form the laminate of the supporting layer:
B: Urethane adhesive layer (4.5 g/m.sup.2 (inner layer))
6: Wrapping sheet of undrawn polypropylene (200 .mu.m thick) (This layer fills the hollow portion between the ridges formed by folding and overlapping the edges of the content protecting layer)
7: Intermediate plastic layer (About 600 .mu.m thick, made of a 1:1 mixture of polypropylene and calcium carbonate.)
8: Top coating layer (10 to 20 .mu.m thick, made of polypropylene block copolymer.)
The transparent food and drink containers which are in current use generally are mostly of a single-layered structure formed from a clear plastic such as polyester, polypropylene, and polystyrene. These containers, however, have poor gas barrier properties and do not preserve foods and drinks for long periods of time. In an attempt to improve the gas barrier properties of these materials, it has been proposed to form the container of a multi-layer structure, with a polyvinyl alcohol layer being interposed between two polypropylene layers. This type of container has the drawback that polyvinyl alcohol loses its gas barrier properties upon moisture absorption. Thus, it is not suitable for long-term storage of water containing foods and sterilized foods.
In another embodiment of conventional plastic cans, the can body incorporates a layer of aluminum foil, which is necessary for gas barrier properties, and a layer of a polypropylene-calcium carbonate mixture. This type of can has an opaque body which obscures the contents of the can. Opaque cans are undesirable containers for those products which must appeal to the consumer's eye. In addition, the conventional plastic cans can not be heated by an electronic oven because the aluminum foil layer reflects the microwave radiation. A need therefore continues to exist for a food and drink, storage and handling container which obviates the above-stated difficulties of conventional containers.